Glaucoma is a disease of the eye characterized at least initially by increased intraocular pressure. On the basis of its etiology, glaucoma has been classified as primary or secondary. Primary glaucoma is an independent syndrome in adults may be classified as either chronic open-angle or chronic angle-closure. Primary open angle glaucoma is the most commonly occurring form of glaucoma where there is no other attributable underlying cause. Angle-closure glaucoma usually afflicts those persons having "shallow" angles in the anterior chamber and results from the sides (or angels) of the chamber coming together and blocking aqueous outflow through the trabecular meshwork. Secondary glaucoma, as the name suggests, results from pre-existing ocular diseases such as uveitis, intraocular tumor or enlarged cataract.
The underlying causes of primary glaucoma are not yet well known. Increased intraocular pressure can be a result of obstruction of aqueous humor outflow. In chronic open-angle glaucoma, the anterior chamber and its anatomic structures appear normal, but drainage of the aqueous humor is impeded. In acute and chronic angle-closure glaucoma, the anterior chamber is shallow, the filtration angle is narrowed and the iris may obstruct the trabecular meshwork at the entrance to the canal of Schlemm. Dilation of the pupil may push the root of the iris forward against the angle or may produce pupillary block and thus precipitate an acute attack of elevated intraocular pressure. Eyes with narrow anterior chamber angles are predisposed to acute angle-closure glaucoma attacks of varying degrees of severity.
Secondary glaucoma is caused by any interference with the flow of aqueous humor from the posterior chamber into the anterior chamber and, subsequently, into the canal of Schlemm. Inflammatory disease of the anterior segment may prevent aqueous escape by causing complete posterior synechia in iris bombe, and may plug the drainage channel with exudates. Other common causes are intraocular tumors, enlarged cataracts, ventral retinal vein occlusion, trauma to the eye, operative procedures and intraocular hemorrhage.
Considering all types together, glaucoma occurs in about 2% of all persons over the age of 40 and may be asymptomatic for years before progressing to rapid loss of vision. It is not clear whether glaucomatous nerve damage is the end result of one pathological process or whether there are several mechanisms by which the final disease is manifest.
There is growing evidence that more than one pathomechanism may be involved early in the glaucomatous process. See for example: Ruben, S. T., Hitchings, et al., Eye 8 (5) pp 516-20 (1994). Among those risk factors are elevated intraocular pressure, family history of glaucoma, age and the vertical cup-to-disk ratio of the internal structures in the posterior chamber of the eye. One study found that in hypertensive eyes without visual field loss, the most important factors in predicting the likelihood of glaucoma-induced loss were the cup-to-disk ratio and age. Johnson, C. A., Brandt, J. D., et al., Arch. Ophthalmol. 113(1) pp. 70-76 (1995). These studies implicitly assume that there are persons who have elevated intraocular pressure (ocular hypertension) without nerve damage to the optic disk or the retina. See also: Pfeiffer N., Bach, M. Ger. J. Ophthalmol. 1(1) pp. 35-40 (1992). Glaucomatous field damage is also known to occur in the eyes of individuals with normotensive intraocular pressure. One theory is that the size of the optic disk determines the susceptibility of the nerve head to glaucomatous visual field damage at statistically normal intraocular pressure. Burk, R. O., Rohrschneider, K., Noack, H., et al. Graefes Arch. Clin. Exp. Ophthalmol. 230 (6) pp. 552-60 (1992). Another explains visual field damage at normotensive pressure as occurring by a different, as yet unidentified, pathologic mechanism. Trick, G. L., Doc. Ophthalmol. 85 (2) pp. 125-33 (1993). Regardless of the theory, glaucomatous visual field damage at statistically normal intraocular pressure is a clinically recognized condition.
Elevated intraocular pressure, while being generally acknowledged as a risk factor for the possible onset of glaucoma, is not a necessary condition for glaucomatous field damage. Nerve cell damage can occur with or without elevated intraocular pressure and nerve cell damage does not necessarily occur in individuals who experience elevated intraocular pressure. Two studies have suggested that increased choroidal perfusion (circulation) may help to prevent glaucomatous optic nerve damage in patients with ocular hypertension. Schmidt, K.G., von Ruckmann, A., et al., Ophthalmologica, 212 (1) pp. 5-10 (1998) and Kerr J.; Nelson P.; O'Brien C., Am, J Ophthalmol., 126 (1) pp. 42-51 (1998). Thus, modernly it appears that glaucoma is characterized as a complex syndrome that manifests itself as optic nerve damage with or without elevated intraocular pressure. It further appears that each symptom, either elevated intraocular pressure or glaucomatous damage to nerve cells, can occur independently of the other. The present invention provides methods to protect retinal ganglion cells and the optic nerve that are damaged or lost despite a therapeutic lowering of intraocular pressure to within normal levels; to protect such cells from damage in the case of so-called normotensive glaucoma; and to protect such cells in glaucomatous eyes that do not respond adequately to treatment modalities intended to lower intraocular pressure.
In cases where surgery is not indicated, topical beta-adrenoceptor antagonists have been the drugs of choice for treating glaucoma. However, alpha adrenergic agonists have more recently been approved for use in the treatment of elevated intraocular pressure and are probably becoming mainstays in the treatment of the disease. Among this class of drugs are various quinoxaline derivatives having alpha.sub.2 agonist activity which were originally suggested as therapeutic agents by Danielewicz, et al. in U.S. Pat. Nos. 3,890,319 and 4,029,792. These patents disclose compounds as regulators of the cardiovascular system which have the following formula: ##STR2##
where the 2-imidazolin-2-ylamino group may be in any of the 5-, 6-, 7- or 8-position of the quinoxaline nucleus; x, y and z may be in any of the remaining 5-, 6-, 7- or 8-positions and may be selected from hydrogen, halogen, lower alkyl, lower alkoxy or trifluoromethyl; and R is an optional substituent in either the 2- or 3-position of the quinoxaline nucleus and may be hydrogen, lower alkyl or lower alkoxy. The presently useful compounds may be prepared in accordance with the procedures outlined by Danielewicz, et al. The contents of both U.S. Pat. Nos. 3,890,319 and 4,029,792 are hereby incorporated by reference in their entirety.
In "Ocular Effects of a Relatively Selective Alpha-2 Agonist (UK-14,304-18) in Cats, Rabbits and Monkeys" [J. A. Burke, et al., Current Eye Rsrch., 5, (9), pp. 665-676 (1986)] the quinoxaline derivative shown below and having the generic name brimonidine was shown to be effective in reducing intraocular pressure in rabbits, cats and monkeys. Compounds in this study were administered topically to the corneas of the study animals. ##STR3##
It has long been known that one of the sequelae of glaucoma is damage to the optic nerve head. The optic nerve head or optic disk is where, along with the retinal vasculature, the axons of the retinal ganglion cell (RGC) bodies that are distributed along the upper layer of the retina converge and are bundled together to transmit signals to the lateral geniculate nucleus. (See diagram of FIG. 6.) Damage to the optic nerve head, clinically referred to as cupping, is observable as areas of depression in the nerve fiber of the optic disk. Cupping is the result of death of optic nerve fibers and alterations in the lamina cribosa, an extracellular matrix that provides structural support. Loss of peripheral vision is a consequence of RGC demise and usually goes undetected until more advanced stages of the disease wherein up to fifty percent of the retinal ganglion cells may already be damaged or lost. Left untreated glaucoma can progress from dimming of vision or loss of acuity to total blindness.
Unfortunately despite long-term lowering intraocular pressure to statistically normal levels by administration of drugs or by surgery to facilitate outflow of the aqueous humor, damage to the nerves in glaucomatous conditions still persists in a significant number of patients. This apparent contradiction is addressed by Cioffi and Van Buskirk [Surv. of Ophthalmol., 38, Suppl. p. S107-16, discussion S116-17, May 1994] in the article, "Microvasculature of the Anterior Optic Nerve". They state:
The traditional definition of glaucoma as a disorder of increased intraocular pressure (IOP) oversimplifies the clinical situation. Some glaucoma patients never have higher than normal IOP and others continue to develop optic nerve damage despite maximal lowering of IOP.
The fact that the nerve damage associated with glaucoma may progress even after significant reduction of intraocular pressure has led many to suggest that pressure-independent causes contribute in many cases. See for example: Schulzer M. et al., "Biostatistical evidence for two distinct chronic open-angle glaucoma populations" Br. J. Ophthal. pp 74916-74200 (1990); Lamping K A, et al., "Long-term evaluation of initial filtration surgery" Ophthalmology 93 (1) pp. 91-101 (1986); Migdal, 1994; Spaeth G L "Proper outcome measurements regarding glaucoma: the inadequacy of using intraocular pressure alone." Eur. J. Ophthal. 6 (2) pp 101-105 (1996). These causes have been suggested to include: (1) induction of apoptosis (programmed cell death) of retinal ganglion cells which is a genetically controlled process whereby unneeded or damaged cells die without eliciting an inflammatory response (see for example: Quigley H A, et al. Invest. Ophth. Vis. Sci., 36 pp.774-786 (1995) "Retinal Ganglion Cell Death in Experimental Glaucoma and after Axotomy Occurs by Apoptosis") and (2) further neuronal degeneration affecting cells (which were not injured by the primary insult) after death or injury of incipiently injured nerve cells. The damage to nerve cells secondary to the primary injury result from overaccumulation of excitatory neurotransmitters released and other noxious environmental conditions created by the death and degeneration of neighboring RGCs. More minor contributors or less understood components in glaucomatous optic neuropathy are: genetic determinants contributing to irregularities in the metabolism of the extracellular matrix and hence susceptibility of the RGCs to damage; vascular compromise which promotes ischemia whether or not related to elevated IOP; and metabolic disorders. Another advantage of the present invention is that it provides a more direct and broader level of protection to nerves because the compounds of the present invention afford protection at the locus of neural damage from both primary and secondary causes.
Retinitis pigmentosa is the term for a group of inherited diseases that affect the retina, the delicate nerve tissue composed of several cell layers that line the inside of the back of the eye and contain photoreceptor cells. These diseases are characterized by a gradual breakdown and degeneration of the photoreceptor cells, the so-called rods and cones, which results in a progressive loss of vision. It is estimated that retinitis pigmentosa affects 100,000 individuals in the United States. The rods are concentrated outside the center of the retina, known as the macula, and are required for peripheral vision and for night vision. The cones are concentrated in the macula and are responsible for central and color vision. Together, rods and cones are the cells responsible for converting light into electrical impulses that transfer messages to the retinal ganglion cells which in turn transmit the impulses through the lateral geniculate nucleus into that area of the brain where sight is perceived. RP therefore affects a different retinal cell type of those affected by glaucoma. Most common in all types of retinitis pigmentosa is the gradual breakdown and degeneration of the rods and cones. Depending on which type of cell is predominantly affected, the symptoms vary, and include night blindness, lost peripheral vision (also referred to as tunnel vision), and loss of the ability to discriminate color before peripheral vision is diminished.
Symptoms of retinitis pigmentosa are most often recognized in adolescents and young adults, with progression of the disease usually continuing throughout the individual's life. The rate of progression and degree of visual loss are variable. As yet, there is no known cure for retinitis pigmentosa.
While not a cure, certain doses of vitamin A have been found to slightly slow the progression of retinitis pigmentosa in some individuals. Researchers have found some of the genes that cause retinitis pigmentosa. It is now possible, in some families with X-linked retinitis pigmentosa or autosomal dominant retinitis pigmentosa, to perform a test on genetic material from blood and other cells to determine if members of an affected family have one of several retinitis pigmentosa genes, and therefore to begin therapy before the damaging effects of the disease become manifest. It is an object of the present invention to protect the photoreceptor cells, the rods and cones by the compounds and methods described herein, particularly in regard to the studies of protection of the photoreceptor cells to light induced damaged by neuroprotective compounds.
Age-related macular degeneration (ARMD) is degenerative condition of the macula or central retina. It is the most common cause of vision loss in the Western world in the over 50 age group. It most commonly affects those of northern European descent and is uncommon in African-Americans and Hispanics. Its prevalence increases with age and affects .sup.15 % of the population by age 55 and over 30% are affected by age 75. Macular degeneration can cause loss of central vision and make reading or driving impossible, but unlike glaucoma, macular degeneration does not cause complete blindness since peripheral vision is not affected. Macular degeneration is usually obvious during ophthalmologic examination.
Macular degeneration is classified as either dry (non-neovascular) or wet (neovascular). In its exudative, or "wet," form, a layer of the retina becomes elevated with fluid, causing retinal detachment and wavy vision distortions. Abnormal blood vessels may also grow into, or under, the retina, creating a neovascular membrane that can leak, further obscuring vision. In advanced cases, scar tissue forms, causing irreversible scotomas, or blind spots. Dry macular degeneration, although more common, typically results in less severe, more gradual loss of vision as one or more layers of the retina degenerates and atrophies. Yellow deposits, called "drusen," or clumps of pigment may appear.
In both forms, the area of the retina affected is the macula (3)--the most sensitive area of the retina. For this reason, people with macular degeneration lose central vision and the ability to see fine detail, while their peripheral vision remains unchanged.
In the case of age related macular degeneration, treatments have been proposed and studied but have found limited success in clinical application. Laser photocoagulation is effective in sealing leaking or bleeding vessels. Unfortunately, it usually does not restore lost vision but only slows or prevents further loss. Conventional laser treatment for exudative macular degeneration generally is effective for a limited amount of time because the abnormal blood vessels tend to grow back.
A newer, investigational approach, photodynamic therapy, has shown some promising results in the treatment of wet (neovascular) ARMD. An injection of a photosensitive dye is given systemically to a patient, which is taken up only in abnormal tissues such as the abnormal vessels present in wet ARMD "A "cold" laser is directed into the eye which activates the dye taken up in the cell walls of the abnormal vessels, thus forming oxidative compounds that lead to clot formation in the neovascular tissues. Fluid leakage is thus halted and as the remaining fluid is reabsorbed, vision improves. Unfortunately, the body also absorbs the clot in 4-12 weeks, so the procedure must be repeated, and, additionally, the laser treatment can cause photic damage to the retina. Another aspect of the present invention is that the compounds of the invention may be administered to protect the retina from damage by the laser light used as a part of this ARMD therapy.
An invasive surgical technique also has been developed that uses specialized forceps to enter into the eye and pull out the neovascular membrane. Unfortunately the neovascularization often grows back.
The cells that nurture the retina, the cells of the retinal pigment epithelium, as well as photoreceptor tissues, have been harvested from human fetal tissues grown in the laboratory and then transplanted. In studies of rats with inherited retinal disease, human fetal retinal pigment epithelium was surgically introduced into the eyes where it functioned normally and restored vision. Unfortunately, transplants in human studies, while initially successful, have failed within three months owing to rejection.
Thus it is evident that there is an unmet need for agents that have neuroprotective effects that can stop or retard the progressive damage resulting from one or more noxious provocations to nerve cells.